The present invention relates to a tappet in which a body is made of light metal such as Al alloy, and in particular, to a tappet in an internal combustion engine, comprising wear resistant material at contact surface with a rotary cam.
To lighten a direct acting type valve operating mechanism in an internal combustion engine, Al alloy tappets are widely used instead of conventional steel tappets. Al alloy tappets are lower than steel alloy tappets in strength and wear resistance, and thus, wear resistant material is applied on the upper surface of a tappet which is slidably engaged with a rotary cam.
FIG. 5 illustrates a conventional Al tappet in which a cylindrical body 21 closed by an upper wall 21a at the upper end is molded from Al alloy. On the upper surface of the upper wall 21a, there is placed a wear resistant cam receiving plate 22 which has a smaller outer diameter than the body 21. The cam receiving plate 22 is fixed on the upper surface of the upper wall 22a by bending an annular protrusion 21b inwardly at the outer circumference of the upper end of the body 21. 23 denotes a rotary cam which contacts the upper surface of the cam receiving plate 22.
The cam receiving plate 22 of the Al alloy tappet acts as a cam follower to a nose 23a of the rotary cam 23. To rotate the rotary cam 23 smoothly, the size or area of the cam receiving plate 22 must be equal or larger than a rotation trace of the nose 23a. The maximum lift length is determined depending on the projected length of the nose 23a from a base circle. Efficiency of intake and exhaust air is determined on the lift length. Thus, to obtain a desired intake and exhaust efficiency, it is necessary to determine the size of the projection of the nose 23a suitably and to keep the diameter of the cam receiving plate 22 in size which is corresponding to the projection of the nose 23a.
As the conventional tappets, since the cam receiving plate 22 is fixed by caulking of the annular protrusion 22b to decrease an effective surface area of the cam receiving plate 22, it is necessary to attach the cam receiving plate 22 having a relatively large diameter in the body 21 having a relatively large diameter to provide a surface area corresponding to the nose 23a of the cam 23. However, the tappet body increases in weight, the cylinder head increases in size, and flexibility in design of the engine decreases.